To be able to produce ever finer structures in the production of semiconductor components with lithographic methods, light of an increasingly shorter wavelength is used. When working in the extreme ultraviolet (EUV) wavelength range, at a wavelength, in particular, between about 5 nm and 20 nm, it is generally no longer possible to work with lens-like elements in the transmission mode. Rather, illumination and projection objectives, or masks, of reflective optical elements are constructed, having reflective coatings adapted to each operating wavelength, on the basis of multilayer systems. Reflective optical elements on the basis of multilayer systems can also be used for the ultraviolet wavelength range.
Multilayer systems are alternately applied layers of a material having a higher real part of the refractive index at the operating wavelength (also referred to as a spacer) and a material having a lower real part of the refractive index at the operating wavelength (also referred to as an absorber). An absorber-spacer pair forms a stack of a certain period thickness. The period thickness is equal to the sum of the individual layers forming a stack. Alternately arranging stacks with an absorber-spacer pair essentially simulates a crystal. Its lattice planes correspond to the absorber layers, on which Bragg reflection occurs. In more complex multilayer systems, a stack may include one or more additional layers besides the absorber layer and the spacer layer. These additional layers may have the function of e.g. avoiding chemical mixing of the spacer material and the absorber material or of enhancing thermal stability of the stacks e.g. in case of heating of the reflective optical element due to infrared radiation.
To be able to image ever finer structures on objects to be exposed, the optical systems of the projection exposure apparatuses desirably ensure the highest possible resolution. This is achieved not only by the use of the smallest possible wavelengths as an operating wavelength, but also by designing optical systems with the largest possible aperture. This has a drawback, however, that both the mean incident angle and the bandwidth of the incident angles can vary widely on individual mirrors as well as from one mirror to another of the optical systems. This can lead to a lower transmission of useful radiation and to imaging defects, leading to a reduced throughput of the projection exposure apparatus, or to an extremely inhomogeneous pupil illumination, so that the imaging quality deteriorates substantially.